The understanding of the solar outer atmosphere requires a simultaneous combination of imaging and spectral observations concerning the far UV lines that arise from the high chromospheres up to the corona. These observations must be performed with enough spectral, spatial and temporal resolution to reveal the small atmospheric structures and to resolve the solar dynamics. An Imaging Fourier Transform Spectrometer working in the far-UV (IFTSUV, Figure 1) is an attractive instrumental solution to fulfill these requirements. However, due to the short wavelength, to preserve IFTSUV spectral precision and Signal to Noise Ratio (SNR) requires a high optical surface quality and a very accurate (linear and angular) metrology to maintain the optical path difference (OPD) during the entire scanning process by: optical path difference sampling trigger; and dynamic alignment for tip/tilt compensation (Figure 2).
The spectroscopy of the far UV emission lines of the solar spectrum combined with an imaging capability is essential to
understand the physics of the outer solar atmosphere. An imaging Fourier transform spectrometer (IFTSUV) is an
attractive instrumental solution to perform such far-UV solar observations. Working in the far UV involves high
precision metrology to maintain the optical path difference (OPD) during the entire scanning process of the
interferogram. It also involves a compact all-reflection design for UV applications. We present the specification of a
servo-system that enables dynamic tip/tilt alignment compensation and OPD sampling measurement of the IFTSUV
scanning mirror. We also discuss the first experimental results of a breadboard as well as the preliminary design of a
space-based device.
Imaging Fourier Transform Spectrometer working in the far UV (IFTSUV) may be the technical solution to
answer many unsolved problems concerning the physics of the solar outer atmosphere. The VUV domain
highly constrains the instruments design and performances as it demands a high optics surface quality and an
accurate metrology to preserve IFTSUV spectral precision and Signal to Noise Ratio (SNR). We present the
advancements on the specification of a metrology system, meeting the predicted performance requirements of
an IFTSUV.
The study of the outer solar atmosphere requires combining imaging and spectroscopy in the UV lines formed
in the high chromosphere, the transition region and the corona. We start from the science requirements and we
define the instrumental specifications in terms of field-of-view (FOV), spatial, temporal and spectral resolution
and bandpass. We propose two different all-reflection optical architectures based on interferometric techniques:
Spatial Heterodyne Spectroscopy (SHS); and Imaging Transform Spectrometer (IFTS). We describe the different
set-ups and compare the potential performances of the two types of solutions, and discuss their feasibility. We
conclude that IFTS appears to be the best solution, meeting the needs of UV solar physics. However, we point
out the many difficulties to be encountered, especially as far as metrology is concerned.
The Molecular Hydrogen Explorer (H2EX) proposed for the 2015 - 2025 Cosmic Vision Call issued by ESA in
2007 was designed to make surveys of the molecular gas from its first rotational lines in various extragalactic and
galactic sites. The design study led to the proposition of a mid-infrared (8 to 29 μm) Imaging Fourier transform
Spectrometer (IFTS) making possible integral field spectroscopy on a 20' wide field and a maximum resolution
up to 3×104 at 10 μm. To reach this goal, an all-mirror payload was outlined, made of a 1.2m telescope matched
to a dual output interferometer, imaging the field on two 1024×1024 Si:As IBC detectors. The payload was
designed to re-use the platform developed for the Planck mission. Such a wide field and high spectral resolution
IFTS on a large spectral domain can have further applications, with the necessary adaptation to each case, for
future large aperture cryogenic telescopes in the mid-infrared, or in the near-infrared behind future ELTs, in a
site like Dome C in Antarctica, and out of astronomy, for remote sensing of Earth atmosphere.
BEAR is a high-resolution imaging FTS in operation on the CFH Telescope, obtained by the coupling of a step-scan FTS and a near IR camera. TIGER is a integral field spectrometer (IFS) for the visible, which was in operation at CFHT, in which a microlens array segments a small entrance field. A new version (OASIS() has been made to be used behind an adaptive optics system, leading to an even smaller field of view. By adapting a TIGER-type field segmentation on an imaging FTS, the instrument remains an IFS more versatile than each instrument taken separately. Such an instrument would be characterized by the access to the same spatial resolution, but on a field larger than with a TIGER instrument, a larger spectral coverage, a continuously chosen spectral resolution as in an FTS, and a better sensitivity than a BEAR instrument. For that, a mosaic of microlens array, made in an IR glass transparent between 1 and 5 microns, is paving one input port of a large field dual-output port interferometer which has a provision for a mirror travel of 5 mm, giving the choice for a resolution up to 104 at 1.7 micrometers . A prism on each output beam disperse the entrance points which are imaged on InSb array. A camera on each output beam records an image at each step of the FTS. This concept can be of interest for the instrumentation of a large ground-based telescope or better for a NGST.
An infrared camera has been successfully coupled to the Fourier Transform Spectrometer (FTS) at the Canada-France-Hawaii Telescope (CFHT), making a unique infrared spectrometer. This instrument, the only one of its kind in the world, is capable of simultaneously acquiring both images and spectra of astronomical sources in the 1.0 to 2.5 micrometers wavelength range across its 24' field of view. Successful observations have been made of various targets, including planets and emission nebulae. A description of the instrument, data analysis, and scientific results, is presented below.
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